This invention relates to an equalization scheme for radio systems, particularly local area networks.
A wireless LAN must provide a fast and reliable radio link between a base station and its terminals. A terminal in a wireless LAN is usually a portable device and therefore it is desirable to reduce the hardware requirements of the terminal as much as possible to make it inexpensive, small, and power efficient. The base station is part of the network infrastructure and usually has access to a reliable power supply so the hardware constraints on the base station are not as rigorous.
When the data transfer rate of the LAN is very high, for example 160 Mbit/s, a characteristic of radio channels called frequency selective fading distorts the signal so much that reliable communications is impossible unless the distortion is corrected. Frequency selective fading occurs because radio waves do not just propagate in a direct path from the transmitter to the receiver, they also reflect off objects in the environment. Reflected signals must travel a further distance than direct signals, therefore they arrive at the receiver later in time. At certain frequencies, the radio waves from the different paths add up in such a way than they cancel each other out almost completely. These frequency xe2x80x9cfadesxe2x80x9d effectively block frequency bands of the radio signal from manking it through to the receiver.
Devices that correct the distortion caused by multipath channels are known as equalizers. Simple linear equalizers that rely on inverting the channel do not work well in radio channels because the deep faded regions of the channel are impractical to invert. More complex non-linear equalizers like the decision feedback equalizer [1] or Thomlinson-Harashima pre-coders [2][3] are better at coping with channel fades. (The reference numerals in square brackets refer to the references listed at the end of this patent disclosure all of which are incorporated by reference herein.) It is also possible to increase the effectiveness of an equalizer by supplying it with signals from more than one antenna [4].
Efforts have been made to reduce the complexity at the terminal by using asymmetrical implementation techniques Gibbard [5] has shown that most of the hardware of a Thomlinson-Harashima pre-coder can be implemented at the base station. Oler [6] has shown that part of the terminal receiver""s decision feedback equalizer can be implemented at the base station to reduce the hardware complexity at the terminal.
This invention integrates the functions of equation, antenna diversity, and frequency diversity with as much of the complex hardware as possible implemented at the base station. The equalization scheme used in this invention is linear which makes it susceptible to failure when there are deep fades in the multipath channel, however, the integrated antenna and frequency diversity allows the system avoid the deep fades altogether which eliminates this problem.
The most challenging part of an asymmetrical equalization system is the downlink to the terminal. Since the terminal is constrained to have little or no signal processing capability of its own, the radio signal must be pre-equalized at the base station. This is done by filtering the signal in the base station so that when it passes through the radio channel, it arrives at the terminal""s antenna undistorted. When part of the spectrum is blocked by a deep multipath fade, generating a pre-equalized signal becomes impractical.
Antenna diversity can help overcome the problem of deep multipath fading. With antenna diversity, radio signals can be rerouted around deep multipath fades so the equation filter does not have to deal with them. To apply antenna diversity asymmetrically, multiple antennas are only allowed at the base station while the terminal retains its single simple antenna. The base station measures the frequency response of each channel during a training period. When the base station transmits back to the terminal along the same channels, it splits its signal between the antennas so that each radio channel only carries the frequency components of the signal where there is no deep fade, and relies on the other antennas to fill in the rest of the spectrum. The components of the signal from the different antennas arrive at the terminal simultaneously and naturally combine to form a complete signal.
Aspects of the invention therefore include:
Implementation of an equalizer for a wireless system that contains very simple terminals by using a linear pre-equalizer for transmitting data to the terminal and a linear post-equalizer for receiving data from the terminal.
Reducing the effect of multipath fading in the radio channel by implementing multiple antennas at the base station and using the linear equalizers to reroute signal energy away from the portions of the radio channels where fading is bad and into different antenna channels where there is less fading.
Reducing the effect of multipath fading in the radio channel by implementing frequency diversity transceivers at the base station and terminals so the linear equalizers can reroute signal energy away from frequencies where multipath fading is bad and into different frequencies where there is less fading.
Implementing frequency diversity transmitter using the expansion operation and implementing the frequency diversity receiver using the decimation operation.
Adjusting the number of frequency diversity channels in the system dynamically by adjusting the parameters of the expansion and decimation operators.
Measuring the impulse response of the channels by sending several copies of the Frank-Heimiller polyphase code from the terminal, sampling the signal simultaneously from each of the base station antennas, removing the first and last cycles of the code, averaging the remaining cycles, and performing a correlation with an expanded version of the original code.
Determining the coefficients of the equalization filters by calculating the matched filters, calculating the response of the system with antenna diversity, calculating the equivalent channel with frequency diversity, solving for the common equalization filter coefficients, calculating the equalization filters as the product of the matched filters and the common equalization filter.
Determining the amount of frequency diversity required to equalize the channel by calculating the frequency response of the equivalent channel with no diversity, finding the deepest fade within that frequency response, while fades that are too deep are detected double the amount of frequency diversity and recalculate.
Implementing a multi-rate DQPSK encoder and decoder that allows the terminal to decode data before it has knowledge of the decimation factor.
Correcting for the channel differences caused by mismatched sample rates between the terminal and base station with a simple comb filter applied at the base station transmitter.